Multiple-site RF transmission systems broadcast signals from more than one base station. This allows radio communications to cover a larger area than is possible with a single base station.
The present invention relates to a network of several single site trunked radio systems. An example of a single site transceiver system is disclosed in commonly-assigned U.S. Pat. No. 4,905,302, entitled "Trunked Radio Repeater System" and U.S. Pat. No. 4,903,321 entitled "Radio Trunking Fault Detection System" which are incorporated by reference. Digital trunked radio transceivers capable of handling communications between numerous mobile units and dispatcher consoles in a single area are known.
It is generally impractical for a single VHF/UHF RF repeater transmitting site to effectively serve a large geographical area. The broadcast area of a single site is limited by several factors. The effective radiated power of the antenna is subject to legal and practical limits. In addition, natural and man-made topographical features, such as mountains and buildings, block RF signal from certain locations.
Multiple transmitting sites are necessary to provide RF communications to all locations within a given locality. Multiple transmitters may be needed to cover a rural community covering many square miles or a city having tall buildings that block RF transmissions. FIG. 1 is a schematic diagram of a simplified multiple-site system having three radio repeater (transmitting) central sites S1, S2, and S3 providing communications to geographic areas A1, A2, and A3, respectively. Mobile or portable transceivers within area A1 receive signals transmitted by site S1, transceivers within area A2 receive signals transmitted by site S2, and transceivers within area A3 receive signals transmitted by site S3. Each site has a site controller that acts as a central point for communications in the site. To enable communications from one area to another, a switch networks the radio systems together to establish audio slots connecting one site controller to another. Thus, a caller in one area can communicate with someone in another area.
The present invention is directed to a multisite RF trunked repeater system that allows a caller in one site area (e.g. A1) to communicate with a callee in another area (e.g. A2). In a multisite network, each site assigns a specific channel to a call independently of the channel assignments made by other sites. Thus, a single call may be broadcast from several site transmitters each operating on a different frequency. A central multisite switch routes audio and command signals from one site to another, and to and from dispatcher consoles.
In multisite, the site controller (S1) receives a call from a mobile radio in A1 requesting a channel to communicate with a specific callee. A caller requests a channel simply by pressing the push-to-talk (PTT) button on his microphone. This informs the site controller that a channel is requested. The PTT signal is transmitted to the unit on a control channel that is continuously monitored by the site controller. The site controller assigns a channel to the call and instructs the caller's radio unit to switch from the control channel to the channel assigned to the call. This assigned channel is applicable only within the area covered by the site.
In addition, the site controller sends the channel assignment to the multisite network switch. The switch assigns one of its internal audio slots to the call. The switch also sends a channel request to all other site controllers or to only those site controllers having a designated callee within its area. Upon receiving a channel request, these secondary site controllers assign a channel to the call. Again, each secondary channel is operative only in the area covered by its secondary site controller. Each secondary site controller also sends a channel assignment back to the multisite switch. The switch connects the site controller line carrying the assigned channel to the assigned audio slot. The caller can then communicate with a unit or group in an other area via the multisite switch. The call is initially transmitted to the primary (host) site controller, routed through the assigned audio slot in the switch and retransmitted by the secondary sites on various assigned channels in those other areas.
When the caller ends the call, the primary site controller deactivates the assigned channel for that site and notifies the network switch that the call is terminated. There may be a brief "hang time" after the end of the call during which the channel remains assigned. During this hang time, the call can be rekeyed without going through the channel assignment procedure.
When the call is dropped, the network switch sends an end of call command to the secondary site controllers. A call is terminated in a similar format and operation as the slot assignment. Instead of establishing an audio slot, the end of call command causes the assigned slots and channels to be released.
In addition to providing communications between mobile radio units in different areas, the multisite network switch provides communications between dispatchers and mobile radio units. The dispatcher consoles are connected to the network switch in the same manner as are the site controllers. A dispatcher console can issue a channel call request through the network switch to a site controller in another area to call a mobile unit or to another dispatcher console to call a dispatcher at another console.
In addition to all of the features that the mobile units have, each dispatcher console has the ability to participate in any call in its area or to its assigned groups. Thus, when a call comes through the network switch from another area to a mobile radio, the network switch informs the dispatcher console of the call in addition to notifying the site controller. The dispatcher can listen in or participate in the call to the mobile radio.
The network switch is also capable of handling calls to groups of mobile units and/or dispatcher consoles. The wide area switch manages group calls and monitors the network to ensure that the site controllers for all of the callees in the group assign a channel to the group call. If a channel is not assigned, the wide area switch advises the caller that the wide area call cannot be formed as requested. The caller then has the option of re-keying the call so as to reach those areas having assigned channels.
The present invention relates to a multisite switch having a distributed architecture. The logical functions of the switch are shared by various microprocessor operated nodes distributed throughout the switch. Each node includes a controller card and audio cards. The nodes share the computational workload of the switch. Within the switch, the nodes are coupled to each other by message and audio buses.
The nodes interface the switch with the other radio system components outside of the switch. Each node is connected to a site controller, dispatcher console, the system manager or other component of the overall radio system. The nodes coupled to site controllers are referred to as Master II Interface Modules (MIMs) and the nodes coupled to dispatcher consoles are referred to as Console Interface Modules (CIMs).
Distributed network multisite systems have a much faster data transfer rate than comparable central architecture multisite systems. Central computers process information serially. All communications passing through the switch must be serially processed by the central computer. The central computer slows communications because of its serial operation. Distributed network systems achieve parallel processing by sharing the computational tasks between several processors. Distributed networks are generally significantly faster than central computers.
There is a tremendous volume of control message traffic within the distributed multisite switch. The bulk of the messages relate to the status of the numerous audio slots within the switch. There are 1024 audio slots on 32 audio buses in the preferred embodiment of the switch. Messages regarding the status of each slot are regularly sent to all nodes over the message bus. Processing these messages could consume much of the processing capacity of the switch nodes, reduce the speed of the switch and degrade switch performance. One purpose of the present invention is to provide a novel method and apparatus that continuously keeps all nodes within the switch apprised of the status of each audio slot, but shields the principal processing unit within each node from redundant slot status messages.
Each node of a multisite network switch includes a controller card operated by microprocessors. The controller cards in each of the nodes have substantially the same hardware and are interchangeable. The MIM and CIM controller cards have identical hardware. Each site controller and each dispatcher console is coupled to a separate node in the switch. Each node acts as a gateway into the network for its site controller or dispatcher console.
The controller card in each node has a communications microcontroller, an interface microprocessor and a dual-port RAM that allows the controller and processor to communicate with each other. The principal logic element of the controller card is the interface processor that assigns audio slots to incoming calls, controls the audio boards in the node, acts on and generates message commands such as slot assignments, slot update and slot idle, performs overhead tasks for the node, and handles other logic functions. The communications controller routes messages passing through the node to and from message bus within the switch, the serial port to the site controller, dispatcher console or other unit, and to and from the interface processor within the node. The communications controller routes and translates messages, but performs few of the intelligence functions of the node. However, one intelligence function performed by the communications controller is to discard redundant audio slot status messages.
When a switch node assigns a call to an audio slot, the interface processor for the node sends a slot assignment message over the message bus to all other nodes in the switch. The slot assignment message identifies the assigned audio slot, the host node and other information. Every other node in the switch reads the slot assignment.
To track the status of each audio slot, each node maintains a bus slot bit map in its dual-port RAM. This bit map has a bit for each audio slot in the switch. By setting and clearing the bits in the map, each node tracks the status of each audio slot in the multisite switch. The interface processor maintains and updates the bit map. The interface processor and communications controller read the bit map to determine the status of the audio slots. If the node is to participate in the call, the interface processor sets the bit in the bit map corresponding to the audio slot identified in the message. The bit for the audio slot is not set if the node is not participating in the call.
After sending a slot assignment message, the primary (host) node periodically sends slot update messages to all other nodes over the message bus. Slot update messages contain the same information as do slot assignment messages, but have a different identification header to distinguish updates from slot assignment messages. These update messages allow other late-coming nodes to participate in the call. A late comer could be a node in which the callee choose to participate in the call after the call began. Update messages also provide fault tolerance to lost or degraded slot assignment messages, or to a node that does not correctly act in response to a slot assignment.
Update messages are sent periodically for all audio slots in the switch regardless of whether the slot is carrying an active call. However, no update message is issued for slots that have not been activated since the switch was powered. Once a slot is activated for the first time, its host node will regularly send update or idle messages for that slot.
Update messages for audio slots carrying active calls are necessary for latecomers and for fault tolerances. Similarly, repetitive slot idle messages are needed to ensure that all audio connections are terminated when completed. A node that wrongly designates an idle slot as being active could force its site controller to maintain an open audio channel for a terminated call. Accordingly, repetitive idle messages are advantageous for the same reasons as are update messages for active slots.
With each node regularly issuing update and idle messages for all its audio slots, most of the messages on the message bus are for updates and slot idles. These messages could overwhelm the processing capacity of the nodes, if the interface processors in each node had to act in response to each update or slot idle message. The interface processor cannot be devoted to processing update and idle messages. The interface processor must be free to perform the important logical and control functions of the node.
The communications controller and the slot bit map in each node shield the interface processor from redundant update and idle messages. Upon receiving a slot update message, the communications controller checks whether the bit corresponding to the identified audio slot is set as being active in the slot bit map stored in its node's dual-port RAM. If the bit is set, then the node already has acted on the call in response to the original slot assignment or to an earlier slot update message. Since the update message is redundant, the communications controller discards the messages without notifying the interface processor. A similar procedure is followed with redundant slot idle messages. The interface processor is oblivious to these redundant messages and, thus, can be devoted to performing the logical functions of the node.